The present invention relates to a safety switching device and method for fail-safely switching off an inductive load, such as a conductor or, in particular, a magnetic valve in a hydraulic press.
Typical prior art safety switching devices are used for switching-off a technical installation, such as an hydraulic press or an automated conveyor system in the event of a hazardous situation, wherein “reliably” means “in a failsafe manner”. For this purpose, inputs of the prior art safety switching devices are connected to signaling devices, such as emergency-off buttons, light barriers, protective doors, rotational speed sensors and others. The safety switching devices evaluate the signals of these signaling devices in a predetermined failsafe manner and, if necessary, they generate a switching signal at their own output, which switching signal, for example, is used to slow down or stop the drive of a monitored system. As a result, a safe state is achieved in a fail-safe manner.
Since health or even the life of operators and others are placed at risk in the case of a failure of the safety switching device, numerous special regulations have to be observed in the development, production and operation of safety switching devices. Accordingly, the present invention is directed to safety switching devices which particularly comply with categories 3 or 4 of the pertinent European Standard EN 954-1 or comparable safety requirements. Within this framework, however, the invention is not restricted to the above-mentioned special examples.
To meet the high requirements for intrinsic fail-safety, typical prior art safety switching devices are often redundantly constructed and/or continuously perform internal functional checks. It is clear, therefore, that the development and production of typical safety switching devices requires considerable effort in comparison with “normal” switching devices. On the other hand, for cost reasons, this increased effort is only made where it is necessary in order to achieve the required fail-safety. For this reason, prior art safety switching devices can contain components and/or parts which by themselves do not have the fail-safety otherwise required.
Prior art safety switching devices are often used for switching off inductive loads, such as contactors or magnetic valves. It is well known that the current flows through such loads cannot be abruptly interrupted due to their inductance. At the instant of switching off, i.e. when one or more switching elements of the safety switching device interrupt the current path to the inductive load, an inductive voltage is produced, the magnitude of which can considerably exceed the operating voltage present at the load under normal operating conditions. It is also known to limit such an inductive voltage by means of suitable threshold switches, such as Zener diodes. Limiting the inductive voltage helps to prevent damage at the output circuit of the safety switching device and at the load to be switched off. Since, on the other hand, the afore-mentioned threshold switches do not have a direct influence on the safety function of the prior art switching devices, it has hitherto not been necessary to provide functional safety measures at this place.
In general, it is desirable to switch off a monitored system as fast as possible when a hazardous situation occurs. This particularly applies to the monitoring of presses since the punch moving down represents a great hazard for the operating personnel, on the one hand, and the operating personnel has to work in close vicinity of the punch on the other.